Despite the origin of pulmonary arterial hypertension (PAH), pulmonary vascular resistance rises due to pulmonary vasoconstriction, arterial remodeling and polycythemia leading to right heart failure and death. Rodents exposed to chronic hypobaric hypoxia (CH) develop PAH. The complex process of developing PAH is driven, in part, by changes in gene expression. In PAH, smooth muscle intracellular Ca2+ is increased and endothelin 1 (ET-1) expression is up-regulated. Ca2+ regulates pulmonary arterial smooth muscle (PASMC) contraction and is linked to gene transcription through the nuclear factor of activated T cells (NFAT). NFATc3 isoform is specifically implicated in the development of the vasculature and maintenance of smooth muscle differentiate phenotype. The overall goal of this proposal is to determine the role of NFATc3 in the molecular mechanisms underlying the vascular changes associated with CH-PAH. The hypothesis is that CH activates NFATc3 in PASMC to mediate hypertrophy and enhance contractility of pulmonary arteries (PA) contributing to PAH. Specific Aim 1: To determine the role of NFATc3 in CH-induced PASMC hypertrophy and PAH. We will estimate PA pressure, measure mRNA and protein of the hypertrophic markers alpha-actin and myosin heavy chain in PA, NFATc3 binding to 1-actin and myosin heavy chain promoters, and determine structural changes of the pulmonary vasculature on wild type +/- calcineurin/NFAT inhibitor and NFATc3 knockout mice exposed to normoxia and hypobaric CH. Specific Aim 2: To establish the contribution of NFATc3 to CH-induced downregulation of Kv channel expression and increases in pulmonary vasoconstrictor reactivity. We will use the same animal models proposed in aim1 and determine mRNA and protein Kv isoforms;NFATc3 binding to KV 1.5 and 2.1 promoters and association to additional transcriptional regulators;PASMC membrane potential and agonist-induced vasoconstriction in isolated pressurized PA. Specific Aim 3: To determine the mechanisms by which CH increases NFATc3 transcriptional activity in PASMC. We will determine the mediators (ET-1, Ca2+, calcineurin and Rho-kinase) of CH-increased NFAT activity using NFAT-luciferase reporter mice and NFAT-luciferase crossed with NFATc3 KO mice. Findings from the proposed studies will provide novel information about the signaling mechanisms regulating changes in gene transcription in PAH. A better understanding of this mechanisms in PAH will lead to the development of novel therapeutic approaches to prevent and cure this debilitating disease.